An MRI apparatus emits a high frequency signal which is an electromagnetic wave to an object disposed in a uniform static magnetic field generated from a static magnetic field magnet. Further, the MRI apparatus excites a nuclear spin existing inside the object, receives a nuclear magnetic resonance signal which is an electromagnetic wave generated from the nuclear spin so as to perform a signal processing and thus image the object. The emitting of the high frequency signal and the receiving of the nuclear magnetic resonance signal are performed by a device called an RF antenna or an RF coil which transmits or receives the electromagnetic wave of radio frequency (RF).
When classified in view of a function of the transmission and reception, the RF coils may be generally divided into three categories, that is, a transmission antenna for performing only transmitting, a reception antenna for performing only receiving, and a transmission and reception antenna for performing both transmitting and receiving. In an MRI apparatus for imaging the human body with a static magnetic field intensity of three or less teslas, a combination is commonly used in which a large transmission antenna, generally of a cylindrical shape or a disk-like shape, is combined with a relatively small reception antenna of various shapes such as a sheet-like shape or a cylindrical shape.
Further, when classified in view of shapes, the RF coils may be largely divided into two categories, that is, one called a surface antenna or a local antenna, and another called a volume coil or a volume antenna. The local antenna is mostly configured to have a circular or planar shape and include a sensitivity function in the vicinity of the antenna so that the antenna is used while being contacted with the surface of the object. On the other hand, the volume antenna has a cylindrical shape or a disk-like shape in which two disks are vertically disposed in a row. Therefore, the volume antenna has sensitivity function inside of the cylindrical shape or in the entire portion between two disks and is thus to be used while an object is disposed therein.
As an example of the volume antenna of the cylindrical shape, there are one called a bird case type (see, for example, NPL 1 and PTL 1), and another called a transverse electromagnetic (TEM) type (see, for example, PTL 2 and PTL 3). In these transmission antennas, typically sixteen to thirty two conductors of a rod-like shape called rungs (cross rod or a cross bar of a ladder) are disposed along the side surface of the cylindrical tube to be parallel to the central axis of the cylindrical tube. Such a transmission antenna of the cylindrical shape is used for an MRI apparatus of a so called tunnel type. In the MRI apparatus of the tunnel type, a magnet of a static magnet field of the cylindrical shape is used to form a tunnel into which the object enters while being laid down on a bed, and thus the imaging is performed.
The TEM type antenna as a transmission antenna of the cylindrical shape is mostly used as a volume antenna for a body trunk or a head in an MRI apparatus having a high magnetic field of three or more teslas. In this case, the currents flowing in the above sixteen to thirty two rungs are coupled with each other so as to be used. Further, the number of independent power feeding points (port) is two.